Merge "Add some comments about thread-safety" am: d0c6e0fa08 am: 8b1745b2eb

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1503391

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: If74f558d5dd3dc64902ea55f8effa626532d4b86